1. Field of the Invention
This invention relates to an attenuator of the type comprising a plurality of discrete attenuation portions which are switched in as desired to provide a chosen attenuation. Such an attenuator is known as a step attenuator and is often used in microwave apparatus.
A step attenuator is a component which can provide discrete attenuation levels where signal levels need to be altered. Such a need may be present in a variety of instruments, such as signal generators, sweep generators and spectrum analyzers.
2. Description of the Prior Art
The fundamental stage of a step attenuator consists of a two state switching mechanism in which one state consists of an attenuating pad and the other a nominally lossless through connection. By connecting any number of these stages in series the desired level of attenuation can be achieved. The attenuation pads are generally constructed from resistive elements such as Tantalum nitride, TaN. They have a maximum power handling capability and there is often the requirement to protect them from overload.
Overload generally arises from the measurement function of the instrument and the protection of the instrument from such overload is often termed Reverse Power Protection, hereinafter abbreviated as R.P.P.
In the past R.P.P. has been provided by a separate component situated between the step attenuator and the test port of the instrument.
Two techniques for R.P.P. are common to the art. Firstly, solid state devices such as limiter diodes have been used in two port circuits which limit the output power as the input power increases. Thus the input power to the step attenuator is limited to a level dependent on the limiter diode circuit. A disadvantage of this method is that the maximum power capability is limited by that of the solid state devices used in the limiting circuitry and the operational bandwidth may be detrimentally affected.
The second technique involves a circuit which samples a constant proportion of the input power to the step attenuator. When a predefined level of power is exceeded, an incorporated switch is open circuited thus disabling the transmission path to the step attenuator. The circuit is constructed as a separate limiting circuit unit placed between the attenuator and the test port of the instrument. The manufacture of such a discrete R.P.P. device is expensive, and causes degradation in performance, and it is desirable to integrate the R.P.P. device into the step attenuator. The present invention provides a method of achieving this.